| |
Okay, when last we spoke we
were reasonably happy with the way our new ship was turning. It grooved
nicely in level flight and was controllable and positive in the glide.
However, you say, line tension isn’t all it should be, especially at 45
degrees and above. What to do?
It's good that you noticed tension is less above level flight. That's a
natural function of gravity and it's going to be a fact of life no matter
what we do. However, we can do some things to minimize the loss.
LINE TENSION (C) in level flight is primarily a function of airplane
weight and speed. Increase either and line tension will increase. The third
most important factor is horsepower. Increase it and line tension will also
increase. Additional horsepower and/or airspeed will increase line tension
in any attitude as they increase Centrifugal Force (someone else can argue
the centrifugal/centripetal force question). I only know if it goes faster,
it pulls harder. On the other hand, as you climb above level flight the
weight of the airplane begins to work against line tension, gravity pulling
vertically down and thus into the flight circle.
Given these three truths, if we want to maintain a greater amount of line
tension above level flight we must either increase airspeed or horsepower to
mitigate the lost tension from the weight vector. Increasing airspeed is not
only undesirable from a pilot’s point of view; it is not possible from a
practical standpoint. The mere fact that increasing lift and climbing
results in increased drag and less airspeed, thus compounding the tension
loss.
You can, however, increase engine horsepower without increasing speed. The
gain in line tension thus achieved will stay with you at any attitude on the
flying hemisphere. We've touched lightly on horsepower in trimming for
response rate. The same factors hold true here. Our two cycle engines, which
we operate at a fraction of their RPM/horsepower potential, can be
configured to operate closer to peak without necessarily resulting in higher
aircraft speed.. The two most obvious methods are increased nitro methane
and/or larger intakes. At the same RPM and thus the same airspeed, the
engine will produce more horsepower and will increase line tension.
Reducing prop diameter unloads the engine and increases horsepower. It may
also increase airspeed very slightly but much less so than the increased
tension would lead one to believe. Reducing pitch also increases the
horsepower; however, it also reduces airspeed and thus is less helpful in
increasing line tension. It will probably result in slightly less tension
unless engine speed is significantly greater.
Finally, Shorter Lines (C-18) will increase line tension if all the above
fails. You should plan on flying with as much line length as your
engine/airplane combo can handle. This allow greater airspeed for the
apparent speed to the pilot (lap time) which means the airplane will fly
happier (faster) and the pilot can't tell the difference.
And that about covers the means of actually "increasing" line tension. We
can, however, do a number of things to ensure that we aren’t throwing away
what we naturally have - Roll Trim (C-2) including Tip Weight (C-3) and Wing
asymmetry C-4); and Yaw Trim (C-5) including Engine Offset (C-6); Lead-out
Position (C-7); and Rudder Configuration (C-8). The worst waster of line
tension is roll. If the airplane is banked toward the pilot, a portion of
the wing’s lift is vectored toward him as well and acts in opposition to
centrifugal force. Any adverse roll must be trimmed out to optimize tension.
Tip weight is the obvious answer. If the amount of required tip weight to
stop the bank is so much that it causes the ship to hinge in hard corners
(drop the outboard wing), consider adding some area to the outboard flap or
removing some from the inboard to make lift increase greater on the outboard
wing with flaps deflected. This situation can usually be avoided if a stunt
ship is built with an additional percent or two of outboard flap area at the
outset.
Yaw, although not as profligate a waster of tension as roll, can be
particularly deleterious above 45 degrees. If the ship flies yawed out
(either from the lead outs being too far aft or from excessive rudder and
engine offset), the fuselage will be forced to fly sideways to the airflow
and will create greatly increased drag. Investigate this possibility by
observing the engine's Oil Pattern (C-17) left on the aircraft after a
flight. The oil should flow mostly down the fuse with only a slight
divergence of the pattern out the wing.
The primary trim control for this problem is the adjustable lead out. If you
have an adjustable rudder, I suggest setting it at a minimum offset until
you find some reason to do something else with it. It is not a particularly
effective tool ...my opinion.
Finally, if after achieving optimum line tension for an airplane, all of a
sudden it starts to slip away from you, don’t start re-trimming. The
airplane most likely hasn’t changed. What has happened is the engine has
gone over the hill. Usually the ring fit has gone to pot. Save yourself a
lot of grief and go to the source. Rebuild it or put in your backup engine.
As a sick engine won’t sound a great deal different from a strong one, this
is often your first due - along with increasing fuel consumption - of a
pooped power plant.
One comment about handles and tension. Longer handle overhang (C-1) will
give increased feel, or perceived tension, while manoeuvring. If your ship
is responding well but you cant "feel” your inputs, try increasing the
overhang.
Let's talk SPEED CONTROL (D) for a minute. A good statement
concerning speed is that the airplane should be flown at the speed at which
it’s happiest and the Line Length (D-6) altered to keep the perceived speed
(lap times) agreeable to the pilot. This isn't always possible due to the
horsepower available, etc. so some other approaches must be considered.
The obvious answer is to increase/decrease the Engine speed (D-1). Remember
that any reduction in engine speed will take it even further off the peak
power curve, therefore, any serious reduction should be coupled with a
consideration of horsepower loss. See the previous comments about line
tension and horsepower.
Propellers (D-2) affect speed ... big surprise! huh. Given adequate
horsepower to turn them, larger diameter props will slow the airplane and
vice versa. More/less pitch will do the same. Again! Remember to consider
the horsepower consequences on line tension.
Here's a cute one for you experimental types. It is my opinion, based on
considerable experimenting, that airplane speed can be controlled by the
pitch of the inner and middle parts of the prop blade. If speed is where you
want it, and horsepower/line tension trim is optimum and you still feel
tension is insufficient above 45 degrees, try sanding more pitch into about
the outer 1/3 of each prop blade. I've done that, especially on the
"Intimidation" with excellent results. If it doesn't work, forget where you
heard it!
Upright/inverted speed (C-7) has been covered many times. They should be
identical and are controlled by raising/lowering the tank/engine
relationship until they are. One trick here for users of Uniflow tanks
systems. If the tank has been moved as far as possible and the run is still
uneven, open up the tank and move the Uniflow vent in the direction you wish
to move the tank. The vent is the tank as far as the engine is concerned,
and moving it has the same effect as moving the entire tank.
At long last, let's discuss the final items of flight trim:
TRACKING (E) and UNIFORMITY OF TURN (F).
All the turn characteristics must be trimmed more or less as we encounter
them and in order of their severity. The classic methods of attacking
unequal rates of turn, inside versus outside, was to either bias the
elevators in relation to the flaps in the direction of the poorer turn, or
less desirable in my mind, to make the handle arm or the line spacing for
the poorer direction longer. I consider both of these methods to be
camouflage and prefer that they be used only as a last resort. In fact, it
is my opinion that biased handle arms will create almost as many problems as
they solve and I would always opt to bias the elevators if it becomes
necessary.
Well, what the heck do I suggest, you ask? First, I suggest making sure that
the wings are level during manoeuvres. If the ship banks out one way, and in
the other, Line tension (F-4) will be better when banked out. Response will
be more positive and the airplane will most probably turn tighter.
Second, I suggest you seal the elevator hingeline (Fl-3). In my experience,
every airplane on which I have sealed the hingeline has become uniform in
rate of turn. The first time I tried it, it was to increase turn rate on the
"Intimidation" which I had been flying two years with about 3/8" of down
elevator cranked in. After sealing, it all trimmed out.
Mis-set handle neutral positions (F-Handle I) will often result in a ship
that appears to the pilot not to turn well in one direction, usually
outsides. This is because the handle is held naturally at neutral with some
down bias. Often trimming gradually back to a vertical neutral will result
in the magical reincarnation of an outside turn.
If all else has failed to make the turns equal, now is the time to bias the
elevator in the direction of the poorer turn. If the airborne controls
aren’t adjustable, you'll have to bias the handle. Make the line spacing
greater for the direction of the poorer turn.
Usually trimming for stability, response rate and turn uniformity will
automatically result in acceptable tracking in manoeuvres. If you still have
problems consider the following: try different props. Props of different
manufacture and even by the same will oftentimes respond differently. Props
which are washed out (less pitch) at the tips will generally track better,
although with sometimes a little loss of tension above 45 degrees.
More flap per degree of elevator will help somewhat with tracking. If you
have only partial span flaps and the problem is severe, I can almost
guarantee that increasing the flap span will improve tracking.
|
|